Service-Tech recently serviced this nasty, mold-ridden plenum box in a commercial facility housing a logistics company were approximately 20 people are employed. Located directly over an employee workstation, the contaminated plenum was very likely emitting mold spores into the air. Instead of ripping out drywall to replace the plenum box, our technicians thoroughly cleaned the plenum box and surrounding area and applied an antimicrobial surface coating for a fraction of the cost. The paint-like coating contains an antimicrobial agent that inhibits the ability of microorganisms to grow. Problem solved.
Have you checked the air source supplies in your facility lately?